Water solutions of alkali silicates are commonly used binders in zinc rich inorganic coatings. Some colloidal dispersions of silica can also be used as binders. Depending on the alkali used for the solution of the silica or the colloidal dispersion and the ratio of alkali to silica these coatings have been classed as either self-curing or requiring a post cure to achieve an acceptable degree of water resistance in say 8 hours.
Examples of these coatings which are commercially available are RUST-BAN 190 (a sodium silicate solution requiring a post curing treatment such as 10% phosphoric acid in water) and RUST-BAN 191 (a sodium-lithium silicate solution which is classed as self-curing). The different cure characteristics of these two coatings is that one (RUST-BAN 190) requires about one month exposure to the atmosphere before it will resist a heavy rain while the RUST-BAN 191 will resist a heavy rain after only about eight hours exposure to the atmosphere depending on the ambient temperature and humidity. Therefore relatively speaking the sodium-lithium vehicle is self-curing.
However, in actual practice, especially in the marine industry which is a major user of these inorganic zinc rich coatings, weather conditions are such that the coating must become completely resistant to rain, fog or extended periods of drizzle in a matter of only one or two hours after application and under less than desirable temperatures such as 50.degree. F and 70-80% relative humidity.
Under these adverse conditions currently available, so called self-curing systems such as RUST-BAN 191 redissolve causing failure of the coating and requiring reblasting and recoating. To offset this problem many end users of these water base inorganic coatings have switched to using the solvent based ethyl silicates because they cure by absorbing moisture from the air. U.S. Pat. Nos. 3,320,082, 3,180,746, and 3,130,061 are relevant prior art patents.